How to assign different background images for different 3d views?
I have attached a file with 3 views. Try hitting Num 7 , Num 1 and Num 3 to view the different elevation views. Everytime you add an image to the background you will have to click on the "Add image" button. I believe that may be the reason why you could not get more than one image to show up on your viewport.





Alternative Methods

However there is a better way of working with image plane. Using import image
as plane. Here you get a polygon created to match the aspect ratio of your image and the image is applied as a shadeless material. This gives you more control in positioning your image planes and make them visible in all views including the 
perspective views.

Note : You may also want to turn on lock transformation for the planes.



Here is the download for the alternative method.